1. Field of the Invention
The present invention relates generally to stimulating medical device, and more particularly, to stimulus timing in a stimulating medical device.
2. Related Art
Hearing loss, which may be due to many different causes, is generally of two types, conductive and sensorineural. In some cases, a person may have hearing loss of both types. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss is often addressed with conventional hearing aids which amplify sound so that acoustic information can reach the cochlea.
In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. Sensorineural hearing loss occurs when there is damage to the inner ear or to the nerve pathways from the inner ear to the brain. Those suffering from sensorineural hearing loss are thus unable to derive suitable benefit from conventional hearing aids. As a result, hearing prostheses that deliver electrical stimulation to nerve cells of the recipient's auditory system have been developed to provide persons having sensorineural hearing loss with the ability to perceive sound. Such stimulating hearing prostheses include, for example, auditory brain stimulators and cochlear prostheses (commonly referred to as cochlear prosthetic devices, cochlear implants, cochlear devices, and the like, and will be referred to simply as “cochlear implants” herein.) As used herein, the recipient's auditory system includes all sensory system components used to perceive a sound signal, such as hearing sensation receptors, neural pathways, including the auditory nerve and spiral ganglion, and parts of the brain used to sense sounds.
Most sensorineural hearing loss is due to the absence or destruction of the cochlear hair cells which transduce acoustic signals into nerve impulses. It is for this purpose that cochlear implants have been developed. Cochlear implants use direct electrical stimulation of auditory nerve cells to bypass absent or defective hair cells that normally transduce acoustic vibrations into neural activity. Such devices generally use an electrode array implanted into the scala tympani of the cochlea so that the electrodes may differentially activate auditory neurons that normally encode differential frequencies of sound.
Auditory brain stimulators are used to treat a smaller number of recipients with bilateral degeneration of the auditory nerve. For such recipients, the auditory brain stimulator provides stimulation of the cochlear nucleus in the brainstem.
In applying electrical stimulation to a recipient, medical devices, such as cochlear implants and auditory brain stimulators, typically use a coding strategy in determining the timing and intensity of the stimulation pulses to be applied. These coding strategies, however, often result in variable latency for received signals. That is, the amount of time taken for a signal being received until the time the corresponding stimulation signal is applied (i.e., used to stimulate the recipient) is variable. In bilateral devices (e.g., where a cochlear implant is used in both errors) this variable latency can result in the loss of phase difference information between signals received at the left and right ears (i.e., the difference in time between when a sound signal is received at one ear versus the other ear). This loss of information may result in poor coding of interaural timing cues for bilateral devices, as well as poor coding of the fundamental frequency for both speech and music. Additionally, typical cochlear implant systems use a fixed pulse rate for applying stimulation. This pulse rate is typically set to a high rate so that the perceived sound doesn't seem less natural and overly robotic to the recipient. This high fixed pulse rate however can result in large power requirements, which reduce the life of the medical devices battery and/or requires larger more powerful batteries. This adds cost, size, and/or inconvenience to the medical device.